The mu opiate receptor (OR) has been identified as the principal brain receptor site best correlated with the rewarding and euphoric properties of opiate drugs. Study of this gene in humans and mouse models will help understand the roles of the gene and its variants in a number of interesting behaviors and other phenotypes. This year investigators in this Branch have continued with vastly-reduced work with mu receptor knockout mice and the human OPRM1 locus due to personnel attenuation. Work on mu knockout mice during this year continued to document modest effects of mu knockout on several intersting features that include hippocampal neurogenesis and ethanol-relted dopamine efflux while studies of strain differences in levels of OPRM1 expression have resulted in identificaiton of a molecular mechanism for these differences that involves an intracisternal particle insertion near this site in an underexpressin strain.